Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
The operating temperature of the fuel cell is high, about 800° C. Therefore, at the time of starting operation of the fuel cell stack, it is desirable to heat the fuel cell stack to a desired temperature rapidly using a combustor. Normally, the combustor is provided on a side of the fuel cell stack where the oxygen-containing gas is supplied, or on a side of the fuel cell stack where the exhaust gas is discharged.
However, in the structure where the combustor is provided on the side where the oxygen-containing gas is supplied, the hot combustion gas produced by combustion in the combustor directly flows into the fuel cell stack. Therefore, the separators tend to be corroded easily by the hot combustion gas, and carbon in the combustion gas adheres to the separators or the membrane electrode assembly.
In an attempt to address the problems, structure of providing the combustor on the side of the fuel stack where the exhaust gas is discharged may be adopted. For example, Japanese Laid-Open Patent Publication No. 2005-166439 discloses a fuel cell system shown in FIG. 10. The fuel cell system 10 includes a fuel cell 1 having an air electrode 1a and a fuel electrode 1b. The air 2 as an oxygen-containing gas is supplied to the air electrode 1a, and a fuel gas 3 is supplied to the fuel electrode 1b. 
On the side of the fuel cell 1 where the off gas is discharged from the fuel electrode 1b, an exhaust gas combustor 4 for burning the off gas is provided, and the combustion gas discharged from the exhaust gas combustor 4 is supplied to a heat exchanger 5, and the air 2 is heated before it is supplied to the air electrode 1a. In a supply line of the fuel gas 3, a start up combustor 6 for supplying the reducing gas produced by incomplete combustion to the fuel electrode 1b is provided.
However, in the conventional technique, since the two combustors, i.e., the exhaust gas combustor 4 and the start up combustor 6 are provided in the fuel cell system, the overall size of the fuel cell system is large.
Further, the exhaust gas combustor 4 is provided on the exhaust gas path of the fuel cell 1, and always exposed to the hot exhaust gas. Thus, durability of the exhaust gas combustor 4 is low. Further, at the time of starting operation of the fuel cell system, since the temperature of the exhaust gas discharged from the fuel cell 1 varies, it is extremely difficult to maintain the temperature of the combustion gas discharged from the exhaust gas combustor 4 in a certain range of the temperature.